Many types of valves and other devices require quarter turn actuation to cause the valve to be positioned such that a flow through a related pipe is regulated or stopped. Numerous styles of quarter turn actuators exist. One of these includes an arcuate piston for pure rotary motion. The majority of quarter turn actuators offered on the market employ dual pistons moving in a linear fashion for force generation and a rack-and-pinion arrangement for conversion of the force and linear motion into torque and rotary motion.
FIG. 1 is a diagrammatic illustration of a prior art pneumatic actuator. As can be seen, the dual pistons 12 and 14 are normally arranged such as to have the same center axis and to travel through a bore opening 16 which continues from one side of the actuator 10 to the other side of the actuator 10 as though it was a continuous cylinder. As the pistons 12 and 14 are aligned on their axis, the rack teeth which engage the pinion gearing 18 are, of necessity, not on the same center axis. Thus, the force F1 generated by pressure acting on the piston 12 is applied to the pinion gearing 18 at a point that is not on the center of the piston area. The piston force F2 thus acts at some distance from the piston center so as to create a moment arm and torsional forces F3 and F4 which cause the piston assembly 12 to deflect into contact with the side wall 16 of the cylinder surface in which the piston 12 travels. This, in turn, causes both ends of the piston assembly to wear against the cylinder wall 16. Suppliers have developed low friction supports for their piston assemblies in an effort to decrease wear and friction. None of these friction supports, however, have eliminated the cause of the problem.
Present actuators, such as that shown in FIG. 1, place the piston seal 20 into the piston 12 itself. As a result, the seal 20 will slide along the finely machined cylinder wall 16. Similarly, the other piston 14 will have a seal 22 positioned within the piston 14. This seal 22 will also slide along the cylinder wall 16. As a result, great machining efforts are required so as to properly fit the seals 20 and 22 into the pistons 12 and 14 and also to form the cylinder wall so as to allow for the proper movement of the pistons 12 and 14 in sealed contact with the wall 16.
Certain actuator suppliers have attempted to arrange their pistons and linkages so that the pistons move toward one another when a valve is being opened. This utilizes the smaller empty volume on the outside of the piston to develop pressures against the piston. This has many disadvantages. Many valve users wish to have the valve fall in the closed position if they should happen to lose their air pressure supply. Normally, a closed valve offers a safer situation when control is lost. Springs are placed into the actuators to force movement of the pistons when air pressure is removed. Unfortunately, all present rack-and-pinion actuators must place the springs on the outer portion of the piston assembly. This requires that the pistons move toward one another to close the valve and away from one another to open the valve. As a result, this maximizes the adverse effect of the empty volume 24 between the pistons.
In addition to the forces caused by the normal actuator having its piston axis offset from the rack-and-pinion gear axis, there exists a second force which causes the rack portion of the piston to tend to move toward the cylinder wall in the interior area 24. This force is that which occurs from the reaction between the rack gearing and the pinion gearing 18. Various means have been employed so as to hold the rack gearing in contact with the pinion gearing. However, the most common method employed is to allow this portion of the piston assembly to rub against the cylinder wall 16 while providing a wear resistance contact surface between the sliding parts.
Many valves require accurate positioning to assure the desired performance. Unfortunately, the design of rack-and-pinion actuators makes it very difficult to incorporate travel position stops in the direction of travel where the pistons move toward one another. It is common to place travel stops on the travel direction that has the pistons moving outwardly. This is easily accomplished with stops in the end caps of the body housing. Most rack-and-pinion actuators place the springs on the outside of the piston, as stated previously, in order to provide spring closure in a failure mode. Under such a circumstance, the pistons must travel toward one another as the valve is closed. Most valves require accurate position settings in the closed position and, thus, the difficulty of incorporating travel stops when the pistons move toward one another is a severe limitation to the application of rack-and-pinion actuators.
It is an object of the present invention to provide a pneumatic actuator in which wear and friction to the cylinder walls are reduced.
It is another object of the present invention to provide a pneumatic actuator that can be more easily manufactured, has lower costs, has faster assembly times, a longer life, and simplified maintenance.
It is another object of the present invention to provide a pneumatic actuator that assures that the pistons are urged to the closed position.
It is another object of the present invention to provide a pneumatic actuator that assures more even and better sealing of the interior of the actuator.
It is a further object of the present invention to provide a pneumatic actuator that provides for easier and safer maintenance.
It is a further object of the present invention to provide a pneumatic actuator that allows for effective travel stop positioning in both directions of piston travel.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.